Some conventional fryers have water and cooking oil stored in a fryer tank in two layers in contact with each other at their boundary, with the water lying at the bottom of the fryer tank and with a heater installed in the oil layer to heat the oil to a predetermined temperature (patent documents 1-3).
In such fryers, during the frying process, minute batter balls dispersing from food ingredients into the cooking oil, colloidal materials drifting away from the food being fried, and water flowing out of the food material (these are hereinafter referred to as impurities) are allowed to sink freely to the bottom of the fryer tank so as to minimize degradation and darkening of oil caused by these impurities.
In other types of fryers, an impurity removing device is provided separate from the fryer and installed in the fryer tank, which has an oil layer and a water layer, to remove impurities such as deep-fried batter balls from the cooking oil by circulating with a pump the oil between the fryer and the impurity removing device (patent document 4).
Patent document 1: Japanese Utility Model Disclosure No. 05-68440
Patent document 2: Japanese Patent Publication No. 55-40249
Patent document 3: Japanese Utility Model Disclosure No. 60-09426
Patent document 4: Japanese Utility Model Publication No. 56-11860
[Problems with Conventional Fryer]
In such a conventional fryer, to prevent a temperature of the cooking oil from falling during cooking and to enable it to return to the set temperature in a short time, the heater disposed in the oil layer is operated at a temperature higher than the cooking temperature of the oil. The oil in contact with the surface of the heater in particular is exposed to a far higher temperature than the set temperature.
Further, since a temperature band of the cooking oil for frying coincides with a temperature band that accelerates degradation and oxidation of the oil, the cooking oil gets easily degraded or oxidized during standby or during cooking. This in turn increases generation of smoke and evaporation, polluting the environment and rapidly changing the composition of the cooking oil.
Thus, there has been a demand for a device that, during standby, can keep the cooking oil at a somewhat lower temperature than that for cooking and, during cooking, quickly raise the oil temperature to the cooking temperature to make it ready for cooking and thereby minimize a quality degradation of the cooking oil.
In such a conventional fryer, if any bubble exists in an oil or water layer and comes into contact with the heated oil, the air bubble will expand explosively, scattering the hot oil.
Thus the bubble in the water or oil layer needs to be eliminated. However, no such device capable of eliminating air bubbles has been available and a fryer with such a function has been called for.
Furthermore, in such a conventional fryer, to remove impurities accumulated in the water layer, it is necessary to open a cock provided below the water layer to drain the water and then fill clean water into the water layer, refreshing the water layer with a predetermined amount of clean water.
Further, in the case of a fryer equipped with a filtering water tank in addition to the fryer tank and using a circulation filtering method, to remove impurities accumulated in the filtering water tank requires manually removing the impurities and cleaning the filtering water tank while not cooking. This takes time, prolonging the cooking interruption time, resulting in a reduced cooking efficiency.
For this reason, there has been a call for a fryer with the filtering water tank that allows easy removal of impurities in the filtering water tank.